Still celebrating National IT Professionals Day with 3 months of free Premium Membership. Use Code ITDAY17

x
?
Solved

ntohl and htonl

Posted on 2009-05-08
6
Medium Priority
?
941 Views
Last Modified: 2012-05-06
In my assignment I am going to send structs over the UDP datagram.. the spec says that everything is in network byte order..

one example of the struct is the following.

So before sending the struct to the other machine, which is a 64 bit machine... what should I do? Should I call htonl?? as I've tried that and the other machine receives the status of the structs differently
typedef struct AARP_Packet {
    u_char			version;		// AARP_VERSION
    u_char			operation;		// See below
    AIP_Addr		aip;		// AIP address
    struct in_addr	ip;			// IP address
    in_port_t		port;		// Port
    u_short			status;			// See below
} AARP_Packet;
 
 
AARP_Packet* temp = malloc(sizeof(AARP_Packet));

Open in new window

0
Comment
Question by:kuntilanak
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 3
  • 2
6 Comments
 
LVL 53

Accepted Solution

by:
Infinity08 earned 2000 total points
ID: 24336096
>> what should I do? Should I call htonl??

Correct.

>> as I've tried that and the other machine receives the status of the structs differently

The other side will need to do ntohl to convert it back from network byte order to host byte order.


A few extra thoughts :

(a) did you make sure that u_short is a 32 bit unsigned integer type on both platforms ? How about all other used types - do they have the same sizes on both platforms ?
(b) did you make sure that there is no struct padding added ?
0
 

Author Comment

by:kuntilanak
ID: 24336178
how can I make sure the u_short is a 32 bit unsigned integer type??

yes, I think the struct is not padded.. AIP_Addr here is just an u_short, it's just another name for it.. so say when I want to send a struct with AIP_Addr of 2, I should do the following:

Is the code below correct?

I've attached a picture on how I laid out the struct


temp->aip = ntohl(2);

Open in new window

pic.JPG
0
 
LVL 53

Expert Comment

by:Infinity08
ID: 24336307
>> how can I make sure the u_short is a 32 bit unsigned integer type??

Did you define it to be a 32bit unsigned integer type ?
How is it defined ?


>> yes, I think the struct is not padded..

If you're not sure, you can't rely on it. The compiler is free to add padding if it thinks that's useful. Some compilers have switches to force it not to add padding.


>> Is the code below correct?

All depends on your struct layout, and the element sizes.


Can you post what the output of this code is for both platforms :
#include <stddef.h>
 
fprintf(stdout, "sizeof(u_char) = %u\n", sizeof(u_char));
fprintf(stdout, "sizeof(AIP_Addr) = %u\n", sizeof(AIP_Addr));
fprintf(stdout, "sizeof(struct in_addr) = %u\n", sizeof(struct in_addr));
fprintf(stdout, "sizeof(in_port_t) = %u\n", sizeof(in_port_t));
fprintf(stdout, "sizeof(u_short) = %u\n", sizeof(u_short));
fprintf(stdout, "sizeof(AARP_Packet) = %u\n", sizeof(AARP_Packet));
 
fprintf(stdout, "offsetof(AARP_Packet, version) = %u\n", offsetof(AARP_Packet, version));
fprintf(stdout, "offsetof(AARP_Packet, operation) = %u\n", offsetof(AARP_Packet, operation));
fprintf(stdout, "offsetof(AARP_Packet, aip) = %u\n", offsetof(AARP_Packet, aip));
fprintf(stdout, "offsetof(AARP_Packet, ip) = %u\n", offsetof(AARP_Packet, ip));
fprintf(stdout, "offsetof(AARP_Packet, port) = %u\n", offsetof(AARP_Packet, port));
fprintf(stdout, "offsetof(AARP_Packet, status) = %u\n", offsetof(AARP_Packet, status));

Open in new window

0
Concerto's Cloud Advisory Services

Want to avoid the missteps to gaining all the benefits of the cloud? Learn more about the different assessment options from our Cloud Advisory team.

 

Author Comment

by:kuntilanak
ID: 24336357
This is what I got on the 32 bit machine and I can't test it on the 64 bit machine, but it can always be assumed to parse it correctly as it's given as a part of the assignment in binary...
sizeof(u_char) = 1
sizeof(AIP_Addr) = 2
sizeof(struct in_addr) = 4
sizeof(in_port_t) = 2
sizeof(u_short) = 2
sizeof(AARP_Packet) = 12

Open in new window

0
 
LVL 53

Expert Comment

by:Infinity08
ID: 24336423
>> but it can always be assumed to parse it correctly as it's given as a part of the assignment in binary...

Ok.

Then you'll need to show a bit more code :)
0
 
LVL 40

Expert Comment

by:evilrix
ID: 24338401
Just as an observation, which I'll let I8 elaborate on if he so chooses (and this isn't mean to contridict anything said so far), generally the safest way to send compound objects across heterogeneous networks is to serialise them first. This removes doubt over things such as packing or data type sizes. It is; however, more work since you need to define a serialization protocol.
0

Featured Post

Concerto's Cloud Advisory Services

Want to avoid the missteps to gaining all the benefits of the cloud? Learn more about the different assessment options from our Cloud Advisory team.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

C++ Properties One feature missing from standard C++ that you will find in many other Object Oriented Programming languages is something called a Property (http://www.experts-exchange.com/Programming/Languages/CPP/A_3912-Object-Properties-in-C.ht…
Basic understanding on "OO- Object Orientation" is needed for designing a logical solution to solve a problem. Basic OOAD is a prerequisite for a coder to ensure that they follow the basic design of OO. This would help developers to understand the b…
Video by: Grant
The goal of this video is to provide viewers with basic examples to understand and use nested-loops in the C programming language.
The viewer will be introduced to the member functions push_back and pop_back of the vector class. The video will teach the difference between the two as well as how to use each one along with its functionality.

705 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question